A plastic optical fiber (below, referred to as an “optical fiber”) is easy to handle because it has a large diameter, and is inexpensive. Thus, it is widely used in telecommunication over short distances. An optical fiber for telecommunications generally is used in the form of an optical fiber cord in which the outer periphery of the optical fiber is covered by a resin. In the case that the optical fiber cord is subject to end surface processing, in the typically used method, the covering material at the end of the optical fiber cord is removed and it is inserted into a plug, anchored by being crimped, and then the optical fiber end surface, which projects about 0.2 mm from the plug end surface, is pressed on a heat plate, and the end surface undergoes a fusion smoothing process. A plug such as F05, F07 that is standardized under ATM standards or the like can also be used.
However, in the case that the optical fiber cord with a plug manufactured by the method described above is used in a severe environment, such as one having a high temperature and a high humidity, fluctuations in form due, for example, to thermal expansion and shrinkage occur in the optical fiber in the plug, and as a result, the protrusion and retraction (pistoning) of the optical fiber from the plug end surface occur. In the case that the protrusion and retraction (pistoning) of the optical fiber from the plug end surface occur, the distance between the light source or the light receiving elements and the optical fiber end surface changes, the amount of received light emitted from the optical fiber fluctuates, and there is the concern that damage will be caused to the system.
In order to solve this problem, in Japanese Patent Application, First Publication No. Hei 7-77642, an optical fiber cord has been proposed wherein, as a primary covering layer, a covering material having a good adhesion with a strand and having a small thermal change covers the optical fiber periphery, and furthermore, as a secondary covering layer, another resin covers the outer periphery thereof. In addition, a method has been proposed wherein, at the end part of the optical fiber cord, the secondary covering layer is eliminated, and the plug is fastened on the primary covering layer.
However, this optical fiber cord has the problem that hydrogen fluoride is generated during baking because a fluorine resin is used as the primary covering layer. In addition, there are the problems that laying the optical fiber cord in a narrow site is difficult because of the lack of flexibility of the primary covering material, and that the cost is high